The plant SDR superfamily: involvement in primary and secondary metabolism.

Short-chain dehydrogenases/reductases (SDRs) comprise a large family of NAD(P)(H)-dependent oxidoreductases represented in plant kingdom by around 190 members in Arabidopsis. Here we will focus on the so-called ‘classical’ SDRs that account for about half of the SDRs and are characterized by a core structure of 250 amino acids and the conservations of specific motifs (cofactor binding site, catalytical residues). Although the ‘classical’ SDR family is highly divergent, an inventory of SDR from the model plant Arabidopsis thaliana genome reveals that it contains both large subfamilies with numerous members that share significant homologies (e.g. tropinone reductase, menthone/salutaridine reductase, NAD-dependent dehydrogenase) and small subfamilies with a limited number of members (e.g. protochlorophyllide reductase, β-ketoacyl-acyl carrier protein reductase and enoyl-reductase). The involvement of SDRs in plant metabolism illustrates the large functional diversity of the SDR superfamily’s members.Functions of SDR encompass many aspects of primary (lipid synthesis, chlorophyll biosynthesis or degradation) and secondary (terpenoids, steroids, phenolics and alkaloids) metabolism. The present paper aims at reviewing the metabolic functions where the role of SDR has been documented and at discussing their phylogenetic diversification and structural features.